Nice write up! I agree with many of your points and I appreciate that you have taken the time and effort to write up your findings. I am curious if you have achieved all of your original goals and requirements? Have you had to change your design in any way? Did you end up with as many smart color bulbs as you originally planned? Or did you end up with more smart switches and dumb bulbs? How is the family liking/adjusting to a smart home?
Curious what you mean by this? Many of us are using Google's Assistant SDK via the Assistant-Relay NodeJS server to be able to Broadcast messages from Hubitat (or ST) to our Google Home devices. It works pretty well as a TTS device.
Or, are you wanting something easier/more direct to control a Google Home device? I completely agree that it would be very nice to not need to run the NodeJS server in order to interact programatically with these devices.